1. Field of the Invention
The present invention relates to an image transmission apparatus for transmitting an image signal and more particularly, to an image transmission apparatus provided with storage means for storing the image signal. The present invention relates also to an image encoding apparatus for transmitting or storing an image.
2. Description of the Prior Art
Of the image transmission apparatus of this type, there is known a facsimile apparatus provided with an image memory.
In the case that the same image data is to be sent to various destinations, or no connection is attained even if transmission to a certain destination is desired, use of the image memory is very advantageous since the image of an original can be stored in the image memory.
However, if the main scan record size or the main scan record density of the destination machine is different from that of the sender machine, e.g., if data of B4 size is stored at the sender and the partner has only record paper of A4 size, then the transmitted image is recorded as partially broken at both ends or one end portion of the record paper, or in some cases it is entirely impossible to communicate.
Also in the case of a different record scan line density, a transmitted image is recorded with an expanded or compressed dimension in the sub-scan direction.
In a facsimile apparatus, the time required for one line recording is decided at the partner machine. Therefore, if a large amount of image data is transmitted at a relatively high speed from the sender, the partner cannot follow such recording. With a definite minimum transfer time for one line image data, it is necessary for the sender to add fill bits as a dummy code to meet the minimum transfer time if one line image data can be transmitted in less than the minimum transfer time.
Storage of such fill bits in an image memory, which bits do not contribute to an image, causes an ineffective usage of the memory capacity. The minimum transfer time varies with the type of the partner machine, so that if fill bits necessary for the longest minimum transfer time are inserted, the amount of image data to be stored in the image memory decreases.
In the facsimile apparatus of this type, a raw image signal read out is temporarily stored in a buffer memory, and the raw image signal in the memory is sequentially encoded if necessary, or the raw image signal is directly transferred if encoding is not necessary. However, if encoding to such as MH code or MR code is performed by means of software, it takes considerable time to encode from the raw image signal.
Of compression-encoding methods for the facsimile apparatus, there are one-dimensional compression such as MH encoding and two-dimensional compression such as MR encoding. The degree of compression is higher in two-dimentional compression than in one-dimensional compression. However, in two-dimensional compression, once an error occurs on one line, then the next line is adversely affected. Specifically, it becomes difficult to read a small character. In recent facsimile apparatus, information regarding the sender and transmission time is transmitted, together with image information, from the sender machine to the receiver machine to confirm at the receiver the sender and transmission time.
Such information is preferably smaller in its printed size than the characters of an original in view of space necessary for such information. However, when an error occurs during transmission in MR code, it becomes impossible to read information printed in a small size.
Conventionally, MR codes have been generated by directly encoding a row image signal. Thus, it becomes complicated to detect, from the raw image signal, inversion of white/black and it takes a long time in encoding.
Conventionally, to store a variable length code and its code length data, it is necessary to prepare a specific number of bits, obtained by addition of the maximum number of bits of the variable length code and the number of bits of code length data necessary for indicating the maximum number of bits.
For instance, the code length of MH code used in facsimile communication is 13 bits at the maximum. The number of bits necessary for indicating the code length is 4 bits, thus necessitating the total of 17 bits.
Therefore, if a 16 bit computer or memory is used, the computer must access the memory twice for encoding or decoding. In addition, as an address of the memory, two addresses are necessary for one code, thus resulting in substantial waste in operation time.